There is much discussion on this forum about sleeping bags and liners. I have done 3 caminos, and backpacking around Asia and Africa for many years. I have always used a silk liner, principally to keep my sleeping bag clean. My 'latest' is 15 years 0ld but this years it ripped, but it did last 14 years without any damage. My sleeping bag is goose down - warmest for weight - and weighs 600 gms. I have walked Caminos twice in Sept/Oct and at first the liner is enough, but by Oct it is getting colder and the bag (with liner) was used. All the contents in my backpack are in waterproof bags, both to keep things dry and for organisation. In my opinion, if there is no need to empty your backpack each night, then you are carrying too much. Every time my pack gets lighter, and this year weighed just under 6 kgs including enough water (600ml). I stop several times each day to fill up with safe drinking water. The longest stretch between water points is from Orisson to the top which is about 14kms, and on the Meseta there is a 17 km stretch. For those I would take a plastic bottle filled with tap water.
Brian B. I have experienced snow in May this year and also in Oct, and rain any time and any place - there are no hard and fast rules although Sept and Oct are reckoned to be the driest times of the year. Buen Camino